Edit: Completely forgot about MNK class restriction on end-game gear.. Well there goes that.
Loved the flexibility in cross-skills that I had as PGL too, so I'm really interested in this as well. The arguments against loosing MNK are Dragon Kick debuff, Demolish for AoE, the 5% Fists of Fire buff, Shoulder Tackle as a gap closer, and a teensy bit of Strength. Now assuming a single target boss fight situation, I don't see any way we could make up for the damage loss from loosing Dragon Kick debuff with cross-skills alone.
However, if there is another MNK in the group keeping up Dragon Kick debuff.. then a lvl 50 PGL build might actually work really well. You would loose the 5% Fists of Fire buff, but both Air and Earth are great alternatives. Loosing Shoulder Tackle as a gap closer hurts, but the distance it covers will save you maybe 3-4s of running every 90s with some extra bit of damage, so I don't think it's a huge deal.
Gaining access to a near passive 10% crit from Archer's Double Tap would be huge asset by itself, not to mention Venom Bite as a powerful extra filler attack (these 2 combined could possibly nearly make up for loosing Dragon Kick when no other MNK in group) along with Raging Strikes + Hawks Eye cooldowns, as Selsix mentioned. Not to mention access to many more
Stacking Internal Release, Blood for Blood, Raging Strikes, Hawk's Eye all at the same time would definitely result in some insane numbers while they are up, for dps races like Titan HM's heart this could be an extremely powerful combo.
I'm really curious and hopeful if this could work, hope some theorycrafters can shed more light on whether or not a level 50 PGL build could actually be viable end-game.